“We are anticipating over 30,000 people being placed in shelters temporarily,” Brock Long, who leads the US Federal Emergency Management Agency, said in a statement Monday morning. “We are expecting over 450,000 potential registers of disaster victims. That is a huge number.”

Harris County, Texas, is one of the hardest hit so far. The region is home to some 2.3 million people in Houston, plus about 4.2 million others in the surrounding area.

Footage of people stuck on rooftops without food, water, shelter, or signs of rescue begs the question: Why weren’t mandatory or voluntary evacuation orders issued for the region ahead of the storm?

“We always say, ‘run from water, hide from wind.’ When we say that, we mean storm surge, not rain,” Harris County Judge Edward Emmett told reporters during a press conference on Friday. “In this case we’ll have a lot of water, but it’s not the kind of water we’ll ask people to evacuate from.”

The biggest reason Houston officials didn’t tell residents to evacuate was to avoid clogging highways and other roads to dangerous levels.

When Hurricane Rita barreled toward Texas in 2005, for example, an exodus of about 3 million people led to 73 deaths prior to the storm (though some estimates claim as many as 107 deaths).

“Traffic jams stretched across hundreds of miles over two days, and many people ran out of gas. Dozens died from accidents and heat-related illnesses, all before Rita even made landfall,” reporters Jim Malewitz and Brandon Formby wrote in The Texas Tribune.

Had Harris County issued an evacuation order even several days in advance, a similar backup may have ensued — and it could have happened on roads that quickly got flooded with several feet of fast-moving water.

As reported by the Tribune and other outlets, most deaths (about two-thirds) caused by flooding happen in motor vehicles. This is because many people drive into what appears to be shallow water on a roadway, only to be swept away due to deceptively strong currents and deeper-than-expected flooding.

Michael Lowry, a scientist with the University Corporation for Atmospheric Research, underscored this point in a tweet on Sunday.

“You literally cannot put 6.5 million people on the road,” Turner said, according to CNN. “If you think the situation right now is bad, you give an order to evacuate? You are creating a nightmare.”

Officials in Harris County did ultimately issue evacuation orders in some areas early Monday morning, according to The Daily Beast. But at this point, many people will not be able to comply with them.

Aisha Nelson, who moved to Houston after Katrina devastated her home, talked about her situation from a rooftop in a Good Morning America live broadcast Monday morning. She said about 30 other people were on the roof with her.

“It’s not going good for us. Across the street the building caving in, and there’s water everywhere,” Nelson said. (One of GMA’s hosts said the program notified the US Coast Guard about Nelson’s location.) “We tried to leave but there was nowhere for us to go. Please help us, I’m scared.”

The full extent of Harvey’s devastation is not yet known, though at least five people have died. At least 50 Texas counties have declared a state of disaster, and insurance research groups estimate that damage may exceed that of Hurricane Katrina in 2005, totaling more than $US100 billion.

Louisiana, meanwhile, is not out of Harvey’s crosshairs. President Donald Trump recently approved emergency declarations for both Texas and Louisiana to direct federal aid toward those states.